Temporary instrumentation, other than dedicated designs that install into the structure of a specific machine, or on a specially designed bracket or boss incorporated into the machine, is usually mounted by clamping, strapping, or adhesive. Designs that are intended to be intermittently used and thus installed and removed as needed are typically mechanically clamped or fixtured to the tested object/machine. FIGS. 1 through 5 depict several devices of this type.
Briefly, FIG. 1 depicts one type of axial extensometer that is used for tensile testing of materials. Blade-type edges are in contact with the test object and elastic bands are used to secure the device to the test object. FIG. 2 depicts a diametric, or radial extensometer that is clamped to the test object. FIG. 3 is another type of diametric test device having a spindle that is used to create a clamping force on the test object. FIG. 4 depicts another prior art device comprising two parallel decks used to measure axial strain on cylindrical shafts. Finally, FIG. 5 is illustrative of a commercially-available extensometer having arms that clamp to a cylindrical shaft; however, this instrument has an offset center of gravity which can be problematic during field testing of equipment.
While each of these devices has been developed for tensile and/or torsional testing, they are either delicate and error-prone, difficult to mount and use, are relatively imprecise, or a combination thereof. This is driven by the nature of these instruments as being temporarily installed, and generally designed for use with a range of test object sizes. For instance, as best seen in FIG. 6, the devices shown in FIGS. 2 through 4 have “vee-block” type distributed contact effects where the instrument has four lines of contact on the test object, each with significant area. This renders the exact gage length, or distance between reference points, indeterminate. It should be noted that strain and deflection-measuring devices typically depend on a precisely defined gage length or reference distance: The precision, stability, and repeatability of the gage length directly limits the performance of the device.
There have been many specialized extensometer designs produced to suit particular applications, often gaining some performance advantages over the generic types, but at the cost of narrow applicability. Yet, there are a number of problems that still exist. First, under vibration or acceleration test conditions, a device may slip or shift. This can cause shifts or other extraneous signals in the output of the device, and produce non-repeatability from test to test. Second, the ability of the instrument to re-acquire a specific mounting relationship on a part for reproducibility from test to test is needed to support calculated and calibrated transfer function equations, and for trending of measured values. That is, when the test object surface is complex or irregular (threads, splines, curved cylinders such as heat exchanger tubes, parts with complex strain fields due to keyways, adjacent fasteners, etc.), consecutive installations often result in the instrument contact areas, and thus the effective gage length, varying from test to test. Third, physical non-linearity and hysteresis (squirm) of the instrument can result. This is out-of-plane motion or distortion of the instrument frame or imperfect tracking of the motion of the test object surface.